Significance: The function of osteocytes and how mechanical strain is translated into biological signals from these cells is unknown. Identification of osteocyte specific proteins that are regulated by mechanical strain and determination of their function should provide significant information regarding the role of osteocytes in the regulation of bone mass. Only recently have osteocyte specific proteins been identified. One of these proteins, called E11, is specifically expressed in the early osteocyte as it is embedding in osteoid and appears to play a role in the generation of dendritic processes. E11 may be essential for osteocyte communication and osteocyte viability. Our hypothesis is that the E11 antigen is responsible for dendrite formation in osteocytes, that allows osteocytes to become embedded in osteoid in a manner to support communication between osteocytes and with cells on the bone surface. Approach: The specific aims for this project are 1). Determine the role of E11 in dendrite formation, osteocyte communication and osteocyte viability, 2). Determine the effects of mechanical loading on E11 expression, and 3). Determine the mechanism of action of E11 through its associations with extra and intracellular molecules known to regulate the cytoskeleton. Cell lines and primary cells overexpressing or underexpressing E11 will be subjected to shear stress to determine effects on gap junctions and hemichannels, major means by which osteocytes communicate. Transgenic mice lacking E11 in osteocytes and mice with reporter molecules driven by the E11 promoter will be subjected to loading and unloading. Innovation: Novel approaches to answer these questions include the use of an osteocyte-like cell line, targeted deletion of genes in osteocytes in vivo, use of endogenous E11 promoter driving a reporter gene for determination of response to loading and unloading, use of siRNA, and the investigation of hemichannels, a novel means by which osteocytes communicate. Therefore we will be able to alternate between in vitro and in vivo approaches for discovery and validation. Investigators and Environment: This project will be conducted as a collaboration between investigators with diverse expertise in academic environments possessing state of the art technology to address the questions posed in this application. Knowledge gained will be applied to the prevention and treatment of bone loss due to immobilization, space flight, microdamage, and disease states such as osteoporosis.